Digital subscriber lines (DSLs) are technologies designed to provide the means for communication over copper wires (twisted pairs, loop) of the existing plain old telephone service (POTS) infrastructure. Such an infrastructure typically includes a central office (CO) employing at least one distribution point (DP) that provides data services to a plurality end-users (i.e., also known as subscribers) that employ devices known as customer premises equipment (CPE) units. The architecture of hybrid communication networks such as very-high-bit-rate digital subscriber line (VDSL) or G.fast (fast access to subscriber terminals) combine the use of an optical fiber segment and a DSL (or G.fast) segment, where the latter stretches along typically the last couple hundred meters over the existing copper wire infrastructure toward the endpoint subscriber. In G.fast, communication equipment is located at the DP and is linked over the communication lines with typically a plurality of corresponding communication equipment (CPEs, e.g., modems) located at the customers' (subscribers') ends.
For proper operation, the modem is generally operative, at least to some extent, to adapt its respective transmission parameters to varying communication line conditions. These transmission parameters have to be known at both ends, so that a receiver will be able to correctly decode received signals from a transmitter. In certain circumstances, however, such as in instances of elevated levels of electronic interference exhibited in the loop, there is a need to change the existing system configuration on-line without causing interruption to the data service; this is what is known as on-line reconfiguration (OLR). Various types of OLR are known, for example, bit swapping, seamless rate adaptation (SRA), transmitter initiated gain adjustment (TIGA), and the like.
Other various methods and protocols for OLR are also known in the art. For example, U.S. Patent Application Publication No. U.S. 2006/0176942 A1 to Oksman et al., entitled “On-Line Reconfiguration and Synchronization Protocol for Multi-Carrier DSL” is directed at a communication system and method for providing an indication of a change in system configuration as a synchronization flag that is temporarily assigned to a plurality of data sub-channels to effectuate an on-line reconfiguration (OLR) of the communication system. The system employs provider and subscriber digital subscriber line (DSL) modems, which are connected via a communication channel and operative to transmit and receive communication signals. The method that is implemented by the system employs a fast OLR procedure that initially identifies free sub-channels that are not being used for data transmission based on a presently used bit loading table configuration. These plurality of free sub-channels are temporarily assigned to carry a synchronization flag, which in turn is transmitted over these sub-channels. The subscriber DSL modem receives and detects the synchronization flag and applies new parameters for subsequent incoming symbols. In order to prevent a possible situation where the synchronization flag is transmitted but not detected at subscriber DSL modem, the provider DSL modem delays the reconfiguration until the subscriber DSL modem sends back a synchronization flag acknowledgement over a sub-channel whereupon the reconfiguration occurs in a synchronized manner between provider DSL modem and subscriber DSL modem.
PCT International Publication Number WO 2011/143101 A1 to Schelstraete et al. and entitled “Systems and Methods for Retransmission with On-Line Reconfiguration” is directed at a system and method for performing retransmission with on-line reconfiguration in DSL systems. The system disclosed, which includes a transmitter and receiver, is a single link retransmission system in which a retransmission method with on-line reconfiguration is implemented. According to this method, a data stream is initially encoded into first frames according to a framing configuration. The transmitter receives a request for an OLR of the framing configuration from the receiver. In response to the request, the encoding of the data stream into the first frames is suspended. The transmitter then enters a retransmission state in which the transmitter transmits one or more first frames transmitted to the receiver during a retransmission time period that commences relative to the suspension of the encoding of the data streams into the first frames. The transmitter then sends an acknowledgement of the OLR request to the receiver. The encoding of the data stream into second frames is resumed according to a modified framing configuration that is consistent with the OLR. The transmitter transmits the second frames to the receiver upon expiration of the retransmission time period.
Reference is now made to FIG. 1, which is a schematic diagram showing an example of a prior art method, generally referenced 10, for enabling synchronization between a receiver and a transmitter via on-line reconfiguration. A horizontal position in FIG. 1 represents information pertaining either to a receiver 12, receiver OLR requests 14, a synchronization status 16, a transmitter 18, and a frame number 20. Frame number 20 identifies and enumerates a frame (frame i, frame i+1, etc.), which is a set of data symbols grouped together that is conveyed between a transmitter 18 and a receiver 12. A vertical position in FIG. 1 represents time in progressing frame units. Receiver 12 is communicatively coupled with transmitter 18, both of which require for synchronization of configuration therebetween. For this purpose, receiver 12 sends to transmitter 18 OLR requests 14, which in turn are to be implemented by transmitter 18. A synchronized state is when both receiver 12 and transmitter 18 employ an identical configuration, such as the same bit loading table (BLT). Synchronization status 16 represents the synchronization status between receiver 12 and transmitter 18.
In an initial state shown in FIG. 1 at frame i, both receiver 12 and transmitter 18 employ the same bit-loading table configuration (i.e., termed “BLT0”) and hence they are synchronized. Suppose, in the upstream (US) direction, at frame i+1, receiver 12 sends an OLR request in the form of message 22 to transmitter 18 for a new configuration having a flat reduction by one bit from the previous configuration (i.e., BLT0). Transmitter 18 receives OLR message 22 and implements the new configuration (i.e., −1 bit) at frame i+4. Now suppose that due to increasing noise conditions receiver 12 sends a new OLR message 24 to transmitter 18 at frame i+5 for a new configuration having a flat reduction of six bits from the previous configuration to be implemented at frame i+7. In this case, however, suppose that that this message (i.e., OLR message 24) does not reach its intended destination (i.e., transmitter 18). At frame i+7 there is a loss of synchronization between receiver 12 and transmitter 18 since they do not employ the same configuration. Moreover, suppose that receiver 12 sends a new OLR message 26 to transmitter at frame i+8 for an additional flat reduction by two bits from the previous BLT and suppose further that this message is successfully received by transmitter 18. At frame i+10 receiver 12 implements the new configuration (i.e., having minus 9 bits), whereas transmitter implements a new configuration that is different (i.e., having minus 3 bits) thereby leading to a long-term loss of synchronization of between transmitter 18 and receiver 12. Since OLR requests are based on the assumption of successful reception and implementation of preceding (i.e., past, “historical”) OLR requests, given a case where transmitter 18 fails to receive one of the OLR messages (e.g., OLR message 24), subsequently received OLR messages (e.g., OLR message 26) accumulate errors resulting in a mismatch of configuration between receiver and transmitter, consequently leading to a loss of synchronization.
Alternative prior art approaches may convey explicitly the absolute bit-loading table per carrier frequency. These approaches will generally be slower reacting (i.e., than the approach described in FIG. 1) as generally there is a need to convey larger amounts of information. These approaches require an acknowledgement based protocol, to make sure that both sides (e.g., transmitter and receiver) implement the newly communicated configuration at the same time.